The guy is wrong. The front sensor should "clock" as the computer
adjusts the mixture. The cat cleans up the exhaust, so the second
sensor only "clocks" if the cat is bad.
I'd suspect either a dirty MAF sensor or dirty injectors - which
techron MAY help. The MAF is fragile, but CAN be cleaned with intake
cleaner and alcohol.

You guys here recently helped me with a check engine light problem I
had with my 96 Lincoln Town Car. (I appreciated the all of the advice,
by the way.)

I had Autozone check the codes and they came up with P0430 (I think),
which indicated a bad sensor--or at worst a bad catalytic converter.

It turns out that when I filled the car with gas the light went out.
Then when I got low on gas again the light went on, but this time it
didn't go back out when I filled it up.

I took it to an electrical specialist, and he put it on the machine
and told me that he got readings from both sensors.  One of the
sensors was flat and smooth and the other one was gyrating all over
the place.  He said that if he didn't get any reading at all then the
sensor might be the problem, but since one sensor was stable and the
other was gyrating that my problem was probably a bad or shot
catalytic converter.  The car has 178K miles on it so he wasn't
surprised.  He said they usually last anywhere from 75 to 100K miles
before having to be replaced.

He reset the check engine light and suggested  I put in some Chevron
Techron additive into the gasoline next time I fill it up.  That could
"possibly" help, but chances are the catalytic converter will have to
be replaced.

It will probably cost anywhere from $400 to $600.

I asked him what would happen if I let it go for a while, and he said
nothing.  It doesn't affect the running of the car at all, but I won't
be able to pass an emissions test.  That's it.  It has nothing to do
with engine performance whatsoever.

Comments?

Thanks,

Tom